The present invention is concerned in one aspect with processes for making hydroxymethylfurfural from sugars, and particularly but without limitation, from hexose carbohydrates such as glucose and fructose. In a second aspect, the present invention relates to the hydroxymethylfurfural products produced by dehydration from such sugars.
Agricultural raw materials such as starch, cellulose, sucrose or inulin are inexpensive starting materials for the manufacture of hexoses, such as glucose and fructose. Dehydrating these hexoses produces 2-hydroxymethyl-5-furfuraldehyde, also known as hydroxymethylfurfural (HMF), among other products such as levulinic acid and formic acid. HMF and its related 2,5-disubstituted furanic derivatives have been viewed as having great potential for use in the field of intermediate chemicals from regrowing resources. More particularly, due to its various functionalities, it has been proposed that HMF could be utilized to produce a wide range of products such as polymers, solvents, surfactants, pharmaceuticals, and plant protection agents, and HMF has been reported to have antibacterial and anticorrosive properties. HMF is also a key component, as either a starting material or intermediate, in the synthesis of a wide variety of compounds, such as furfuryl dialcohols, dialdehydes, esters, ethers, halides and carboxylic acids. A notable example of a compound that can be prepared from HMF is 2,5-furandicarboxylic acid, or FDCA, which can be prepared from HMF, ether or ester derivatives of HMF through an oxidation process, see, for example, U.S. Pat. No. 7,317,116 and US 2009/0156841 to Sanborn et al. FDCA has been discussed as a biobased, renewable substitute for terephthalic acid, in the production of such multi-megaton polyester polymers as ethylene terephthalate or butylene terephthalate. FDCA esters have also recently been evaluated for replacing phthalate plasticizers for PVC, see, e.g., WO 2011/023491A1 and WO 2011/023590A1, both assigned to Evonik Oxeno GmbH, as well as R. D. Sanderson et al., Journal of Appl. Pol. Sci. 1994, vol. 53, pp. 1785-1793.
In addition, HMF has been considered as useful for the development of biofuels, fuels derived from biomass as a sustainable alternative to fossil fuels. HMF has additionally been evaluated as a treatment for sickle cell anemia. In short, HMF is an important chemical compound and a method of synthesis on a large scale to produce HMF absent significant amounts of impurities, side products and remaining starting material has been sought for nearly a century.
While it has correspondingly long been known that HMF can be prepared from sugars through dehydration, being initially prepared in 1895 from levulose by Dull (Chem. Ztg., 19, 216) and from sucrose by Kiermayer (Chem. Ztg., 19, 1003), chemists have differed over the years as to the precise mechanisms by which HMF is formed from certain sugars. As related very recently in Weingarten et al., “Kinetics and Reaction Engineering of Levulinic Acid Production from Aqueous Glucose Solutions”, ChemSusChem 2012, vol. 5, pp. 1280-1290 (2012), “[o]verall, there are two schools of thought with regard to the mechanism of HMF formation from C6 carbohydrates. One theory postulates that the reaction proceeds by way of the acyclic 1,2-enediol intermediate. The other takes into account a fructofuranosyl cyclic intermediate in the formation of HMF from fructose.” In relation to glucose, specifically, Weingarten reports that there are likewise two theories for how HMF is formed from glucose: “One theory suggests that the formation of HMF from glucose proceeds via fructose and that the near-nil presence of fructose can be attributed to its high reactivity compared to glucose. Conversely, other authors claim that glucose can be converted directly to HMF through cyclization of a 3-deoxy-glucosone intermediate formed from the open-ring form of glucose. In this respect, the relatively low conversion of glucose to HMF is caused by its low affinity to exist in the open-ring form due to stabilization of the glucose pyranose forms in aqueous solution.”
While there accordingly seems to be no overriding consensus as to the precise manner in which HMF and other observed dehydration products are formed in the dehydration of hexose carbohydrates such as fructose and glucose, yet there is nevertheless a consensus that whatever mechanisms may be at work and whatever intermediate species may be formed by such mechanisms, a number of unwanted side products invariably are produced along with the HMF—whether through reactions involving the intermediate species or involving HMF—so that an economical process to make HMF on a large scale with good yields has not yet been realized. Complications arise from the rehydration of HMF, which yields by-products, such as, levulinic and formic acids. Another unwanted side reaction includes the polymerization of HMF and/or fructose resulting in humin polymers, which are solid waste products. Further complications may arise as a result of solvent selection. Water is easy to dispose of and dissolves fructose, but unfortunately, low selectivity and increased formation of polymers and humin increases under aqueous conditions.
The realization of an economical commercial production of HMF has also been hindered by HMF's comparative instability and tendency to degrade, so that purification of the HMF from the various side products and from unconverted sugars has itself proved difficult. On long exposure to temperatures at which the desired product can be distilled, for example, HMF and impurities associated with the synthetic mixture tend to form tarry degradation products. Because of this heat instability, a falling film vacuum still must be used. Even in such an apparatus, resinous solids form on the heating surface causing a stalling in the rotor and frequent shut down time making the operation inefficient. Prior work has been performed with distillation and the addition of a non-volatile solvent like PEG-600 to prevent the buildup of solid humin polymers (Cope, U.S. Pat. No. 2,917,520). Unfortunately, the use of polyglycols leads to the formation of HMF-PEG ethers.
Still other more recent efforts to deal with HMF's comparative instability and tendency to degrade have sought to either form more stable and easily separated HMF derivatives, for example, HMF ester and ether derivatives, or to quickly remove the HMF from exposure to those conditions, for example, acidic conditions, tending to contribute to its degradation.
An example of the former approach may be found in the previously-cited US 2009/0156841 by Sanborn et al., in which a method is provided of producing substantially pure HMF and HMF esters from a carbohydrate source by contacting the carbohydrate source with a solid phase catalyst; “substantially pure” was defined as referencing a purity of HMF of about 70% or greater, optionally about 80% or greater, or about 90% or greater.
A method of producing HMF esters from a carbohydrate source and organic acids involved, in one embodiment, heating a carbohydrate starting material with a solvent in a column, and continuously flowing the heated carbohydrate and solvent through a solid phase catalyst in the presence of an organic acid to form a HMF ester. The solvent is removed by rotary evaporation to provide a substantially pure HMF ester. In another embodiment, a carbohydrate is heated with the organic acid and a solid catalyst in a solution to form an HMF ester. The resulting HMF ester may then be purified by filtration, evaporation, extraction, and distillation or any combination thereof.
An example of the latter approach may be found in WO 2009/012445 by Dignan et al., wherein HMF is proposed to be made by mixing or agitating an aqueous solution of fructose and inorganic acid catalyst with a water immiscible organic solvent to form an emulsion of the aqueous and organic phases, then heating the emulsion in a flow-through reactor at elevated pressures and allowing the aqueous and organic phases to phase separate. HMF is present in the aqueous and organic phases in about equal amounts, and is removed from both, for example, by vacuum evaporation and vacuum distillation from the organic phase and by passing the aqueous phase through an ion-exchange resin. Residual fructose stays with the aqueous phase. High fructose levels are advocated for the initial aqueous phase, to use relatively smaller amounts of solvent in relation to the amount of fructose reacted.
In WO 2013/106136 to Sanborn et al., we described a new process for making HMF or HMF derivatives (e.g., the ester or ether derivatives) from an aqueous hexose sugar solution in which, according to certain embodiments, the acid-catalyzed dehydration step is conducted with rapid heating of the aqueous hexose solution from an ambient to a reaction temperature, as well as with rapid cooling of the HMF and/or HMF derivative unconverted sugar mixture prior to the separation of the fermentation-ready residual sugars product from the HMF and/or HMF derivative product. In addition, the time between when the aqueous hexose solution has been introduced into a reactor and the HMF and/or HMF ether products begin to be cooled is preferably limited.
By accepting limited per-pass conversion to HMF, the overall exposure of the HMF that is formed from any given aqueous hexose solution to acidic, elevated temperature conditions is limited, and preferably little to no unwanted or unusable byproducts such as humins are produced requiring waste treatments. Separation and recovery of the products is simplified and levels of HMF and other hexose dehydration products known to inhibit ethanol production by fermentation are reduced in the residual sugars product to an extent whereby the residual sugars product can be used directly for ethanol fermentation if desired. Processes conducted as described were characterized by very high sugar accountabilities and high conversion efficiencies, with very low losses of sugars being apparent.